Traditionally, baseband to radio frequency (RF) up-conversion is performed in the analog domain with one of three types of architectures: heterodyne; super-heterodyne; or direct conversion.
In heterodyne or super-heterodyne architectures the baseband signal can be either complex, utilizing analog quadrature modulation, or real, utilizing digital quadrature modulation, for modulating the baseband signal to a low intermediate frequency. In direct conversion architectures the baseband signal is complex and utilizes analog quadrature modulation.
Analog up-conversion and previous attempts at digital up-conversion have generally been frequency dependent, that is the up-conversion techniques operate over a small frequency band. Therefore, the same design will not operate effectively at other frequency bands. For example, an up-converter designed for operation at 800 MHz will not operate properly at 450 MHz, 1.5 GHz or 1.9 GHz.
One disadvantage of analog up-conversion is that it is subject to issues related to analog variability such as, but not limited to, component to component variability, temperature variability, voltage variability and variability due to aging. Another disadvantage of analog up-conversion is inflexibility, as an analog up-conversion architecture needs to be designed for specific frequency bands. Therefore, it is not possible to easily convert a radio for use in an alternate frequency band without physically replacing an up-converter in the radio, as well as other components that may be frequency dependent, with a newly designed up-converter for the alternate band, or have multiple up-converters each capable of operating in a different frequency band. Either option adds an increase to the cost of a system.